An operational amplifier is a circuit unit that possesses a high amplification factor. In an actual circuit, an operation amplifier often forms a functional module together with a feedback circuit. Operational amplification can be achieved by an independent device or in a semiconductor chip. For a transistor amplifier, the amplifier may be a class A amplifier, a class B amplifier, a class AB amplifier or a class D amplifier.
Taking the class D amplifier for example, as shown in FIG. 1, an output structure of Bridge Tied Load (BTL) can output four states formed by high level and low level, namely, high-high, high-low, low-low and low-high. When the BTL outputs are the high level or low level at the same time, high level common mode signals or low level common mode signals may occur. In order to achieve high performance, it is inevitable for the amplifier to use feedback common mode signals. A formula for calculating the value of feedback common mode (VFCM) signals is: VFCM=(OUTP+OUTN)/n, where OUTP is the sum of all feedback signals at a BTL input positive end, OUTN is the sum of all feedback signals at a BTL input negative end, n is the number of feedback circuits, and OUTP and OUTN may be either high level AVDD or low level AGND.
FIG. 2 shows an operational amplifier circuit with feedback signals. An output Pulse Width Modulation (PWM) signal is directly transmitted back to the input ends of the operational amplifier. Because the PWM signal has very strong common mode signals, the input ends of the operational amplifier will have very strong common mode signals too. Also, OUTP and OUTN may be either high level AVDD or low level AGND, so that the value of the feedback common mode signals of the operational amplifier circuit is AGND, AVDD or AVDD/2. The feedback common mode signals of the circuit are as shown in FIG. 3.
After analyzing the conventional art, the inventors find that: because the feedback signals of an operational amplifier circuit have relatively strong common mode signals, if the feedback signals are directly transmitted back to the input ends of the operational amplifier circuit, the input ends of the operational amplifier circuit will have very strong common mode signals too. In order to guarantee the normal operation of the operational amplifier circuit, the operational amplifier circuit must have a wide common mode input range and have a high common mode rejection ratio. Otherwise the performance of the circuit will be lowered.